1. Field of the Invention
The present invention relates to the field of wireless communication, and more particularly, a method of channel estimation and compensation based thereon.
2. Description of Related Art
In recent decades, many new technologies, such as multi-carrier transmission and smart antenna algorithms, have been proposed to increase the capacity of multi-user wireless communication systems. However, the performance improvement promised by such new technologies is usually limited by the accuracy of channel estimation. Channel estimation is the estimate of the distortion between the transmitter and the receiver of a signal introduced by the physical channel or medium through which the signal was transmitted. Using an estimate of this distortion (i.e., channel estimate), the receiver can remove some of this distortion and improve the accuracy of the received signal. Even a small improvement in channel estimation may introduce significant benefit to, for example, multi-user technologies.
Because of the movement of a mobile station in a wireless communication system, the physical channel is constantly changing. If coherent detection of the distortion in the physical channel is desired, then the phase of the distortion must be tracked. Because of the difficulty in tracking the phase of the distortion, the current code-division multiple access CMDA standard in the United States, IS-95, uses non-coherent detection on the system uplink (communication to the base station). Non-coherent detection is estimating data symbols in the received signal without accounting for distortion in the physical channel.
However, non-coherent detection suffers from poor performance with respect to the signal-to-noise ratio SNR, and SNR performance is directly related to the capacity of CDMA systems. Thus, the next generation CDMA standard, CDMA2000, has proposed coherent detection on the system uplink. To accomplish coherent detection, a pilot signal has been proposed for use in the system uplink. Accordingly, using the pilot signal, a simple form of coherent detection, called pilot assisted detection, can be performed.
Let the general received signal at the base station from a given mobile station be represented as:
r(t)=xcex1(t)[p(t)+jxc2x7s(t)]+n(t)xe2x80x83xe2x80x83(1)
where xcex1(t) is the time-varying complex channel distortion assumed to be a complex Gaussian random process (GRP), s(t) is the data signal of interest, p(t) is the unmodulated pilot signal and n(t) is the noise plus interference which is assumed to be a GRP. Further, let p(t) and s(t) be defined such that they are orthogonal over some predetermined interval, Tj, i.e.,                                                         ∫              0                              T                i                                      ⁢                                          p                ⁡                                  (                  t                  )                                            ⁢                              s                ⁡                                  (                  t                  )                                            ⁢                              xe2x80x83                            ⁢                              ⅆ                t                                              =          0                ⁢                  
                ⁢        and                            (        2        )                                                      ∫            0                          T              i                                ⁢                                                    "LeftBracketingBar"                                  p                  ⁡                                      (                    t                    )                                                  "RightBracketingBar"                            2                        ⁢                          xe2x80x83                        ⁢                          ⅆ              t                                      =                  ϵ          p                                    (        3        )            
where xcex5p is the normalized pilot energy over Ti.
Assuming that the channel is constant over the interval [0, Tp], i.e., xcex1(t)=xcex1,                                                                         α                ⋒                            =                              xe2x80x83                            ⁢                                                1                                      ϵ                    p                                                  ⁢                                                      ∫                    0                    Tp                                    ⁢                                                            r                      ⁡                                              (                        t                        )                                                              ⁢                                                                  p                        *                                            ⁡                                              (                        t                        )                                                              ⁢                                          xe2x80x83                                        ⁢                                          ⅆ                      t                                                                                                                                              =                              xe2x80x83                            ⁢                              α                +                N                                                                        (        4        )            
where * signifies the complex conjugate, {circumflex over (xcex1)} is the initial estimate of xcex1 and   N  =            ∫      0              T        i              ⁢                            p          *                ⁢                  (          t          )                    ⁢              n        ⁢                  (          t          )                    ⁢              xe2x80x83            ⁢              ⅆ        t            
is a complex Gaussian noise term.
Having obtained the channel estimate, an estimate ŝ (t) of the data signal s(t) is then obtained. The data signal is defined as:                               s          ⁡                      (            t            )                          =                              ∑                          i              =                              -                ∞                                      ∞                    ⁢                                    d              ⁡                              [                i                ]                                      ⁢                                          m                T                            ⁡                              (                                  t                  -                                      iT                    s                                                  )                                                                        (        5        )            
where d[i] is the data symbol during the ith symbol interval and mT(t) is the pulse waveform defined on [0, Ts] , where Ts is the time duration of one symbol, such that equation (2) is satisfied. The data symbols are estimated as:                                           d            ⋒                    ⁡                      [            i            ]                          =                  f          ⁡                      (                          ⁢                              {                                                      ∫                                                                  (                                                  i                          -                          1                                                )                                            ⁢                                              T                        s                                                                                    iT                      s                                                        ⁢                                                            r                      ⁡                                              (                        t                        )                                                              ⁢                                                                  m                        T                        *                                            ⁡                                              (                        t                        )                                                              ⁢                                                                  α                        ⋒                                            *                                        ⁢                                          xe2x80x83                                        ⁢                                          ⅆ                      t                                                                      }                                      )                                              (        6        )            
where ƒ represents a decision function based on the modulation scheme, for example ƒ(x)=sgn(x) in bi-phase shift keying (BPSK), and xcfx84(x) is the imaginary part of x. The estimates in equation (6) are the detected data in the pilot-assisted detection scheme.
As discussed previously, in view of the significant benefit obtained from even small improvements in the channel estimation, a demand exists for improvements on the pilot assisted detection methodology.
The method of channel estimation according to the present invention makes an initial channel estimate using a pilot signal in the received signal and generates an estimate of the data in the received signal. Another channel estimate, is then obtained from the estimated data. This process of estimating the data and re-estimating the channel estimate is reiterated a desired number of times to refine the accuracy of the channel estimate.
Additionally, in a further embodiment of the present invention, after generating a channel estimate from the estimated data, an adaptive channel estimate is obtained. The adaptive channel estimate is generated based on the two most recently generated channel estimates. The degree of adaptation may be determined to further improve the accuracy of the channel estimate and reduce signal-to-noise ratio in the wireless communication system to which the method according to the present invention is applied.